The association between gestational body mass index change, cesarean section, and other pregnancy outcomes: Research protocol: Retrospective cohort analysis.

Increasingly the value of gestational weight gain (GWG) as a useful measure to predict obstetric outcomes including cesarean section is being questioned. We will undertake a retrospective cohort study of recorded births in 2023 from 24 to 42 weeks' gestation for women aged from 16 to 49 years in the United States. Using multivariable Robust Poisson Regression, maternal body mass index (BMI) and BMI gain will be modelled on the primary outcome of cesarean delivery. The primary analysis will be restricted to nulliparous women if there is interaction between parity and BMI or BMI gain. A sample of 716,392 births will be required to show approximately a one percentage point increase in cesarean section for each BMI classification compared with normal BMI (18.5 to <25) or per unit gain in maternal BMI. We hypothesize that the risk of cesarean section will increase with an increase in the BMI category and that BMI at birth may be a more relevant predictor of cesarean section than pre-pregnancy BMI. This question is important to answer given recent evidence casting doubt on the association between excessive GWG, which has been the focus of most interventions (with modest impact at best), and adverse pregnancy outcomes.